The proposed research consists of the isolation and identification of the androgens secreted by mammalian fetal testes which determine male phenotypic development of the Wolffian and Muellerian ducts, and urogenital sinus derivatives. The experimental design consists of the sequential (a) establishment of a bioassay for androgen activity, based on explanted ducts in organ culture and morphologic and metabolic criteria, (b) demonstration of androgenic activity in culture media of cultured fetal testes and (c) isolation and identification of the active androgens; i.e., presumably a C19 steroid for Wolffian duct stabilization, and a macromolecule inducing Muellerian duct regression. As part of this study, the metabolism and uptake of testosterone by reproductive ducts will be examined to determine the role of the ducts in their own differentiation. Ancillary experiments will include inhibitors of androgen synthesis and action, in vivo studies in rabbit fetuses and chick embryos, and the role of adrenal androgens, of mesonephroi, and of ductal metabolism in ductal differentiation. In addition, the non-testicular sources of C19 steroids will be studied systematically.